1. Field Of The Invention
The invention relates generally to the art of gas analysis and more particularly is directed to a new metabolic rate analyzer.
2. Description Of The Prior Art
There are many different techniques for gas analysis in the prior art. In the field of metabolic rate analysis, various gas analysis techniques are used to measure the amount of oxygen (O.sub.2) and carbon dioxide (CO.sub.2) in the mixture of gases inspired and expired by the subject being studied. Such techniques are used by physicians for clinical reasons and by athletes and coaches to measure fitness levels.
It has long been known that the analysis of a subject's respiratory air provides valuable information relating to the physical condition of the subject. The four most commonly-measured variables are respiratory volume, oxygen consumption, carbon dioxide production and respiratory exchange ratio (RQ), which is the ratio of carbon dioxide produced to oxygen consumed. The earliest efforts to conduct metabolic rate analysis involved the use of a so-called Douglas bag. The Douglas bag metabolic analysis technique involved the timed collection of expired breath in a rubberized breathing bag, measuring the volume of expired gas collected and analyzing the gas composition contained within the rubberized bag for O.sub.2 and CO.sub.2 content. Metabolic rates were then calculated from the data obtained. The Douglas bag technique was time consuming, subject to error and could only be performed on relatively stationary subjects in well-equipped laboratories. Also, this technique was not well-suited to the measurement of short duration transients in metabolic functions.
Since the data obtained from respiratory gas analysis is so valuable in diagnosing cardiopulminary dysfunction and evaluating overall cardiovascular fitness, intense effort has been directed towards the development of simpler, faster, automated metabolic rate analyzers. The intense interest in physical fitness and aerobic exercise, such as running, has helped to focus further effort in this field. Many instruments are presently available for the determination of the total volume of expired air from a subject being studied. These devices include, for example, spirometers, plethysmographs and pneumotachographs. Numerous instruments are also available for determining O.sub.2 content and CO.sub.2 content in expired gas. Some of the more recent techniques found in the prior art involve the use of a discrete zirconium oxide O.sub.2 sensor and a non-dispersive infrared (IR) gas analyzer for determining CO.sub.2 content. Such instruments are accurate, however they require frequent calibration and special operating skills. Moreover, such prior art devices, especially those that provide accurate results, are costly and cumbersome. Normally, such instruments can only be used in a clinical or laboratory environment with a subject that is in a resting, or basal, condition or a subject that is confined to a stationary exercise device, such as a treadmill or stationary bicycle. Despite the intense interest now in physical fitness, there seem to be no instruments for determining metabolic rate that may be taken with an athlete, such as a runner or bicyclist, for the purpose of providing a measure of metabolic rate under transient conditions while the athlete is engaged in his normal exercise regime.
Measurement of metabolic rate is also useful in critically ill patients for the purpose of providing an indication of relative changes in cardiovascular function, or such other physical characteristics as tissue profusion.
Separate from the measurement of metabolic rate, CO.sub.2 analysis in the expired gases of an anesthetized patient has evolved into a recognized technique for monitoring the viability of the anesthetized patient. Such CO.sub.2 analyzers and gas analysis techniques relate generally to the art of capnography. The more recent capnography instruments conduct breath-by-breath CO.sub.2 analysis in the end tidal gases expired from an anesthetized patient with a non-dispersive IR gas analysis technique.
Non-dispersive IR gas analysis is also the technique for CO.sub.2 analysis preferred in the most recent metabolic rate analyzers. Non-dispersive IR gas analysis involves the application of infrared energy to a sample of the expired gas and the measurement of IR attenuation in the sample due to absorption of infrared energy by CO.sub.2. The infrared energy applied to the sample is confined to a narrow bandwidth in which it is known that CO.sub.2 has a high absorptivity for infrared energy and attenuation in the sample must be compared to a reference. While these instruments are portable in the sense that they can be carried from place to place, they are relatively expensive and bulky and they have substantial power requirements which make them unsuitable for use, for example, by an athlete during his regular training regime or by a cardiac patient during rehabilitative therapy.
Another problem encountered in prior art techniques for metabolic rate analysis and CO.sub.2 gas analysis involves the presence of water vapor in the expired gas. Gaseous samples of inspired air typically have a water vapor partial pressure of about zero to twenty-five torr. Moreover, in patients receiving ventilatory support in which the inspired gas is humidified, the water vapor pressure typically varies between zero and about forty-seven torr. Gaseous samples of a patient's expired gas typically have water vapor partial pressure of about forty-seven torr. Water vapor interferes with the operation of many gas analysis techniques used in the prior art, thus requiring that water vapor in the sample be minimized or removed. In doing so, errors can result from the concentrating effect of the gases being measured. Detrimental effects of water vapor on respiratory gas analysis stem from the fact that the partial pressure of the water displaces the analyzed inspired or expired gas, introducing inaccuracy. Another associated problem is the error introduced if the water vapor concentration in the inspired gas is not equivalent to the water vapor concentration in the expired gas. Equalization is often required to cancel out the affects of water vapor when the O.sub.2 or CO.sub.2 concentrations of the inspired and expired gases are compared. Techniques used in the prior art to remove water vapor from the sample gases, such as physically drying the gases, introduce other problems related to the condition and efficacy of the desiccating agents and the volume of the desiccator which provides increased dead space within the system and results in a longer sample time or wash-out time for measuring changes in gaseous composition.
The concept of using permeable membranes for the separation of gases and vapors dates to the nineteenth century. This technique is based on the selective permeability of certain organic materials. The term "selective permeability" as it is generally used in the art means that one gas in a mixture of gases will permeate through a membrane faster than the other gases in the mixture. It should be understood, however, that the term "selective" does not necessarily imply the passage of one gas to the complete exclusion of others. The result is always that a gas mixture on the high pressure side of the membrane is depleted in the concentration of the more permeable component, just as the gas mixture on the low pressure side of the membrane is enriched in the more permeable component. It will be apparent from the description of the present invention that, as used herein, the term "selective permeability" does not require that the gas of interest in a mixture of gases will permeate faster than the other gases in the mixture, it only requires that a significant rate of permeability is present for the gas of interest.
In selective permeable membranes, gas dissolves in the membrane on the side having a high partial pressure, diffuses through the membrane under the influence of the pressure difference, then comes out of solution on the low pressure side. This mechanism can result in large differences in permeation rates for the same gas through different polymeric membranes and a considerable spread in permeabilities for different gases in a given polymer. It has been known for some time that dimethyl silicone and many other silicone derivatives have oxygen or carbon dioxide permeabilities considerably higher than most permeable non-silicone plastics. This rapid transport is thought to be a result of the very flexible silicone-oxygen-silicone chain in the absence of crystallinity in silicone rubber. In addition to providing high permeability, such silicone membranes also demonstrate high separation factors for different types of gases, i.e., ratio of permeabilities.
Since the transport of gas or vapor, including water vapor, depends only on the partial pressure difference of the gas or vapor across the membrane, it is known that transport of a given gas or vapor, such as water, can be blocked by saturating the low pressure side of the membrane with the gas or vapor. For example, if one side of the membrane is in contact with liquid water, there will be no partial pressure driving force across the membrane, and water will not be transferred across the membrane.
Such selectively-permeable membranes have found many uses in the prior art. These include heart/lung machines where the membranes are used to separate blood from oxygen, separation and enrichment of gases such as to produce enriched air in an oxygen tent for emphysema therapy, air supply systems for space craft, underwater craft or divers, wound dressings and other barriers for passing certain gases but also filtering bacteria, pollen, dust and other contaminants, as well as in the instrumentation for gas analysis. In the instrumentation for gas analysis, such selectively-permeable membranes have been used to contact blood and other liquids for the purpose of measuring carbon dioxide levels in the liquids. In such cases, the CO.sub.2 diffuses through the membrane and is then swept or purged into an instrument for conducting CO.sub.2 analysis. Such selectively-permeable membranes have also been used in oxygen analyzers; for example, in enriching feed streams to gas chromatographs, and in controlling the feed streams to mass spectrometers.